The invention relates to an apparatus for measuring the area (i.e. printing area) of portions of a printing plate to which ink is applied during a printing process the measurement being carried out before the printing process so as to previously adjust the ink quantity fed to said printing plate.
The printing machines such as lithographic offset press have generally plural devices (so-called "ink fountains") that are adapted to control the ink supply rates and are disposed in a direction rectangular to the rotational direction of a printing plate attached to said machines. At initiation of the printing process, an operator should adjust the ink supply rate with regard to each ink formation by controlling the opened extent thereof so that an optimum lateral distribution of the ink quantity can be attained. In other words, the ink quantity distribution has hitherto been adjusted manually be the operator so as to avoid any excess or shortage in respect of each lateral portion of the printing plate formed thereon with an irregular pattern of figures that are to be printed on papers or the like. Such adjustment of ink fountains takes however a comparatively long period of time. Moreover, it is very difficult even for a skilled operator to exactly adjust the ink supply rates of all the ink fountains.
It has, therefore, been required to execute the initial adjustment more quickly and more accurately by any reasonable or convenient means in order to shorten the time in making necessary preparations for printing process as well as to reduce the paper loss in such adjustment work. To this purpose, it would be effective to previously measure the area of residual photosensitive layer on the printing plate such as a lithography plate. Said area of the printing plate is an image or picture portion thereof which is to be supplied with the ink, and the area will hereinafter be called "printing area". Some apparatuses have already been developed to meet the abovesaid requirement and a few of them are now used practically.
One of the known measuring apparatuses utilizes a television camera as disclosed in the U.S. Pat. No. 3958509 while in the other known apparatuses an optical system comprising some lenses is combined with photosensor elements as shown in the Japanese Early Publication Gazette (Patent) No. Sho. 50-17278 and the U.S. Pat. No. 4239393. These known systems are however not so advantageous in view of a wide variety in the size of printing plates because the size of optical system or the range of vision (or the resolving power) of said camera should be great enough to measure the biggest plate having for instance a width of more than 1 meter as well as a length of more than 1 meter. Besides such great dimension of the apparatus, a higher degree of precision will also be needed in designing such apparatuses thereby resulting in a higher cost in manufacture thereof.
To solve the above problems, there is proposed a simpler and less expensive apparatus as schematically shown in FIG. 1. This apparatus does not include any complicated optical system of lenses but has a photoelectric scan head 3 comprising beam sources of light (not shown) illuminating a printing plate 1 and a row of photosensors 2.sub.1, 2.sub.2 . . . 2.sub.n respectively disposed in the head relative to each of said sources. According to this intricate apparatus, intensities of the light beams reflected by the printing plate 1 are photoelectrically measured to determine the printing area of said plate. An example of such apparatus is disclosed in the prior Japanese Patent Application No. Sho. 56-93208 (hereinafter called "J.P.A.56-93208") in the name of the applicant of the present application.
In a practical use of such photoelectric apparatus, a calibration is indispensable to obtain correct data of the printing area. The calibration factor may in turn be obtained by testing output levels of the photosensors sensing on one hand the printing plate portion having a "printing area ratio" of 0% and on the other hand the portion having said ratio of 100%. The above "printing area ratio" is defined as a quotient of the "printing area" divided by a unit area including same wherein said "printing area" per se has the meaning as mentioned above. The so-called "calibration mark" or "100% patch" (hereinafter called "calibration mark") having the printing area ratio of 100% is therefore provided at a suitable location on the printing plate in such a manner as to permit the operator to carry out the calibration of said apparatus. (Another calibration as to the printing area ratio of 0% is possible without employing any additional calibration mark because such portions having said ratio of 0% always exist near the edges of said printing plate.) For example, a further Japanese Early Publication Gazette (Patent) No. Sho. 56-24508 has disclosed a calibration mark that is provided on a printing plate and is adapted " to calibrate the measured intensities of light reflected by the upper level and the lower level." On the other hand, the aforecited apparatus in the J.P.A. 56-93208 necessitates a calibration mark on a printing plate in order to calibrate or correct the sensitivities of its plural photosensors as well as to adjust the electric gains of its amplification circuits, each of which is connected with one of the photosensors. Details of the system disclosed in the J.P.A. 56-93208 are as follows. Namely, signal levels of each photosensor are at first measured with regard to the abovesaid portion or calibration mark which respectively have the printing area ratio of 0% or 100%. (The signal levels are hereinafter called "0% signal level" and "100% signal level", respectively.) Then, the 100% signal level is subtracted from the 0% signal level whereby the thus obtained difference is used to calculate "ratios of actual signal insensities" to said difference during measurement of the printing area ratios described hereinbefore. Thus, the sensitivities of each photosensors are calibrated simultaneously with said measurement.
To put it concretely, an absolutely reflective (white) portion and an absolutely absorbent (black) portion or calibration mark thereof should preliminarily be measured with all the photosensors. Said portion and said mark are uniform in their reflective character over their whole range. FIG. 2 illustrates a series of output voltages for the white portion at 7 together with another series of output voltages for the black mark at 8, respectively with regard to each photosensors 2.sub.1, 2.sub.2 . . . 2.sub.n. One of the sensors, for instance 2.sub.1, is thereafter selected to be a representative sensor which should measure the intensities of beam reflected by the white portion and the black calibration mark. The thus obtained 0% signal level and 100% signal level are shown in the FIG. 2 at 9 and 10. As to remaining photosensors 2.sub.2 through 2.sub.n, said signal levels 11 and 12 are calculated by means of the above data shown at 7 through 10. It will now be apparent that such an apparatus shown in J.P.A. 56-93208 requires a printing plate formed with a calibration mark having the printing area ratio of 100%, in a manner as exemplified in FIG. 1.
In general, almost all the known apparatuses inclusive of the just described one do utilize such (a) calibration mark(s) formed on the printing plate and having said ratio(s) of 100% (and 0%). This will result in a complication and a poor productivity in the plate making process.